For example, as disclosed in PTLs 1 and 2, the aforementioned power module includes a power module substrate obtained by bonding a metal plate serving as a circuit layer to one surface of an insulating substrate, and a power device (semiconductor device) mounted on the circuit layer.
In addition, a heat sink such as a radiator plate or a cooler is arranged on the other surface of the power module substrate to radiate heat from the power device (semiconductor device) in some cases. At this time, in order to reduce the thermal stress caused by the difference of thermal expansion coefficients of the insulating substrate and the heat sink such as a radiator plate or a cooler, the power module substrate is configured such that a metal plate serving as the metal layer is bonded to the other surface of the insulating substrate and the metal layer and the aforementioned heat sink such as a radiator plate or a cooler are bonded to each other.
In the aforementioned power module, the circuit layer and the power device (semiconductor device) are bonded through the solder material.
Here, when the circuit layer is composed of aluminum or an aluminum alloy, for example, as disclosed in PTL 3, it is necessary to form a Ni plating film on the surface of the circuit layer by electroplating or the like and to arrange the solder material on the Ni plating film to bond the semiconductor device to the circuit layer.
In addition, when the circuit layer is composed of copper or a copper alloy, a Ni plating film is formed on the surface of the circuit layer and the solder material is arranged on the Ni plating film to bond the semiconductor device to the circuit layer.